This invention relates to a laser having at least one optical stage.
In a conventional laser there is a single optical stage and this is the oscillator stage. The oscillator stage has an active element formed from active material and means for pumping the active material. Under the influence of the associated pumping means, the active element suffers from a lens effect and this lens effect arises for the following reason. Most of the light supplied by the pumping means and incident on the active element is dissipated in the active element thereby causing the temperature of the active element to rise. Because the outside of the active element is cooled, there is a temperature gradient across it, with the highest temperatures occurring along it axis. Because the optical length of the active element varies with temperature, the result of the temperature gradient is to cause the active element to behave as if a positive lens were located at a central position. Such a lens will be referred to hereinafter as a thermally induced lens.
In a laser in which there is no compensation for the thermally induced lens, the beam divergence varies with input power. For many applications of a laser, the output beam from the oscillator stage can be coupled, in a satisfactory manner, into the next optical element by placing this next optical element at a suitable distance from the output mirror.
For some applications of a laser, it is required that the output beam is collimated. In order to obtain a collimated beam, it is known to include optical elements at fixed positions inside the cavity of the oscillator stage so as to compensate for the thermally induced lens. In some lasers, the average power of the pumping means can be varied so as to match the requirements of the operation being performed. This variation in average power causes a variation in the focal length of the thermally induced lens. Known arrangements for compensating for the thermally induced lens suffer from the disadvantage that they are designed to provide compensation at a particular average power of the pumping means.
Lasers have been proposed which have one or more amplifier stages following the oscillator stage. In such a laser, the thermally induced lens in the active element of the amplifier stage causes the output beam of the amplifier stage to converge. The angle of convergence varies with the average power of the pumping means associated with the amplifier stage. The convergence of the output beam of the amplifier, and more particularly variation in the angle of convergence, makes it difficult to couple the output beam of the amplifier stage into the next optical element.